The present invention generally relates to electrical signal filters, and more particularly relates to filtering electromagnetic interference from an input/output port such as a universal serial bus port.
Input/output (I/O) interfaces allow various computing devices such as computers, peripherals, digital cameras, handheld devices and the like to communicate with each other. I/O interfaces typically include one or more data ports that receive cables or other connecting elements, along with associated electrical and signaling codes for data signals passing through the data port. Common interface standards used by the computing industry include Universal Serial Bus (USB), IEEE 1394 (xe2x80x9cFirewirexe2x80x9d), Ethernet, small computer system interface (SCSI), serial, parallel and others. One version of the USB standard, for example, is described in the Universal Serial Bus Revision 2.0 Specification dated Apr. 27, 2000 and available from the Universal Serial Bus Implementers"" Forum of Fremont, Calif.
As computing speed continues to increase, I/O interface designs continue to evolve to support new technologies and ever-increasing data throughput rates. Faster data transfer rates, however, frequently result in additional design concerns. In particular, the faster signaling rates used in high-speed data ports often produce undesirable electromagnetic interference (EMI) and other noise in transmitted signals. Various schemes have been devised to reduce or remove noise, typically using one or more inductive filters to remove EMI. As data transfer speeds continue to increase, however, a need arises for increasingly sophisticated filters to remove higher frequency noise from transmitted signals.
Filter designs are often further complicated by the need for backward compatibility with older equipment. The USB version 2.0 standard, for example, requires support for devices based upon the older USB version 1.1. This backward compatibility requires that USB version 2.0 devices (which primarily transmit data using differential signals) also utilize the single-ended signals used by USB version 1.1 implementations, particularly the xe2x80x9cend-of-packetxe2x80x9d (EOP) waveform that is used to indicate the end of a data transmission. If the receiver does not properly receive the EOP signal, transmitted data can run together and/or become corrupted. In practice, however, the high-attenuation filters typically used for high-frequency data transmissions have been observed to distort the EOP signal such that it is not properly recognizable by the receiving device. Accordingly, the amount of attenuation that can be used in a high-speed data filter is limited by the amount of distortion produced in low-frequency or single-ended signals transmitted through the filter. This presents a significant design challenge, since the reduced attenuation generally results in increased noise and/or EMI present in the transmitted signal.
Accordingly, it is desirable to create a filter for an input/output (I/O) port that provides improved noise attenuation while effectively transmitting both differential and single-ended signals. In addition, it is desirable to create a filter that is easily implemented and that does not unduly increase the size of the I/O port. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.